The invention relates to an electric discharge machining process. More particularly the invention relates to a dielectric fluid which is applied in the process to both a workpiece and an electrode. The invention is also a dielectric fluid.
2. Description Of The Related Art
The electric discharge machining process is a method of cutting and working metals which are difficult to form by more conventional metal forming processes. The process is used to shape extremely hard brittle metals into precision parts.
In the electric discharge machining process a cathodic electrode is brought into close proximity with the workpiece which is the anode. Cathode and workpiece are submerged in a dielectric fluid with a predetermined gap maintained between the two. A voltage is applied in an amount exceeding the breakdown voltage of the dielectric fluid and a series of arcs are struck across the gap through the fluid at a frequency of 20,000 to 300,000 per second. The electric arc erodes the surface of the workpiece. By this erosion the workpiece assumes the shape of a mirror image of the proximate surface of the cathode. By selection of the cathode surface shape, corresponding surfaces are shaped on the hard brittle anode which cannot practically be obtained by other machining processes. The method is particularly effective for making small cavities, piercing, trepanning and similar operations on tools and dies. For example, precision square holes can be formed to a preselected depth.
The quality of the machined article can be improved by flushing the anode surface with the dielectric fluid to remove debris generated by the electric arc erosion. The stream of fluid is directed at the point from which the electric arc is generated for greatest effectiveness. This results in a rapid degradation of the fluid. Improvements to the process have come by way of improvements in the degradation stability of the dielectric fluid.
Dielectric fluids are known in the art. Dielectric fluids which have been used for electric discharge machining include paraffinic mineral oil, naphthenic mineral oil, kerosene, glycols, silicone oils and water. The suitability of a fluid for use in the process is judged primarily on the insulating dielectric barrier provided by the fluid between the electrode and the workpiece; and on the ability of the fluid to pass high-peak discharge currents when ruptured at breakdown voltage. The dielectric fluid must also be relatively non volatile at operating temperature; have a low viscosity for fluidity, particle settlement and convective cooling. The fluid must retain these characteristics even under degradation in the environment of an electric arc. Accordingly, there is a need in the art for a dielectric fluid which is of high dielectric strength, is non corrosive, is free of objectionable odor, evolves minimal smoke and is particularly resistant to oxidation.
Phenolic compounds have been used in fluids for their oxidative stability. For example, 1,6-hexamethylene bis(2,6-ditertiarybutyl-4-hydroxyhydrocinnamate) is offered for sale as an antioxidant under the tradename IRGANOX.RTM. L109 by Ciba Geigy Corporation. The manufacturer suggests this compound for combination with synthetic or mineral oils to formulate compressor oils, turbine oils, FDA white oils and grease.
U.S. Pat. No. 4,444,676 to G. L. Stratton et al. discloses phenolic antioxidants for polyoxyalkylene polyether polyols and polyurethane foams prepared therefrom. The phenolic antioxidants include 1,6-hexamethyl bis(3,5-ditertiarybutyl-4-hydroxycinnamate).
U.S. Pat. No. 4,427,563 to D. A. Hutchison discloses phenolic antioxidants for stabilizing hydrocracked lubricating oils. For example, 4,4'-bis(2,6-ditertiarybutyl phenol) is employed to inhibit deterioration due to light.
Electric discharge machines and operating conditions are described in U.S. Pat. No. 5,051,554 to A. T. Tsukamoto; U.S. Pat. No. 5,041,709 to R. J. Schneider et al. and U.S. Pat. No. 4,717,803 to J. Alexandersson all incorporated herein by reference.